Cascade Process\nfor Coupling Molecular H<sub>2</sub>‑free Hydrogenation of\nCO<sub>2</sub> with Alkylation of Aromatics\nUsing a Single-Bed Catalytic System

A single-step, molecular H<sub>2</sub>-free, integrated\ncatalytic\nprocess for producing xylene-rich alkylated aromatics (AAs) using\nCO<sub>2</sub> as an alkylating reagent has been developed, wherein\nmethylcyclohexane (MCH) is used as a liquid organic hydrogen carrier\n(LOHC) as well as a reactant, and CO<sub>2</sub> as a carbon source\nto generate alkylating species. A multifunctional catalyst has been\ndeveloped through the metal-functionalization of zeolite. Physico-chemical\nproperties of the catalyst were studied using characterization techniques,\nlike XRD, FTIR, NH<sub>3</sub>-TPD, pyridine-IR, N<sub>2</sub>-adsorption–desorption,\nICP-OES, UV–vis absorption spectroscopy, HR-TEM, HR-SEM, TGA,\netc. The coexistence of Lewis and Bro̷nsted acid sites in a\nsingle catalyst facilitates three types of reaction in a single pass,\ni.e., the dehydrogenation of MCH to produce toluene and hydrogen,\nthe hydrogenation of CO<sub>2</sub> to form active alkylating species,\nand the alkylation of toluene to produce AAs. Using LOHC not only\nsidestepped the storage, transport, and safety issues associated with\nthe molecular H<sub>2</sub>, but also conferred almost four times\nhigher product yield and carbon utilization compared to the molecular\nH<sub>2</sub>-based process. In a single pass, >70% MCH conversion\nand ∼58% (wt/wt) selectivity for AAs in the liquid product\nwere achieved, which is equivalent to the space–time yield\nof ∼390 mg g<sub>cat</sub><sup>–1</sup> h<sup>–1</sup>. The catalyst could effectively suppress the toluene disproportionation\nand the methanol-to-olefins reaction. The values of reaction free\nenergy calculated using the DFT method indicate that the LOHC-based\nroute is thermodynamically more favorable than the molecular H<sub>2</sub>-based route. Moreover, the in situ utilization of the dehydrogenated\nspecies of LOHC as a reactant makes the process atom-economical and\nevades the process complexity associated with the separation of dehydrogenated\nLOHC.